Method of gravel packing a well and product formed thereby



Jan. 9, 1968 .1. l.. Hul'r'r ETAL 3,362,475

l METHOD OF GRAVEL PACKING A WELL AND PRODUCT FORMED THEREBY Filed Jan.1l, 1967 2 Sheets-Sheet 1 INVENTORS .//MM/E 1.. ,uu/7'7- BRUCE a. M.-aoroz//V Jan. 9, 1968 J. l.. HUITT ETAL 3,362,475

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United States Patent O 3,362,475 METHOD F GRAVEL PACKING A WELL ANDPRODUCT FORMED THEREBY `limrriie L. Huitt, Glenshaw, and Bruce B.McGlothlin, OHara Township, Allegheny County, Pa., assignors to GulfResearch & Development Company, Pittsburgh,

Pa., a corporation of Delaware Filed `lan. 11, 1967, Ser. No. 613,718Claims. (Cl. 166-20) ABSCT GF THE DHSCLOSURE A method of gravel packinga well to prevent flow of formation sands into the Well in whichsubstantially spherical glass particles are packed in -a cavitysurrounding the well bore in the producing formations penetrated by thewell. The size of the glass particles in the glass pack decreases as thedistance from the borehole increases. The density of the liquid used tocarry glass particles into place in the gravel pack is controlled to aidin deposition of the particles against the wall of the cavity.

This application is a continuation-in-part of Ser. No. 413,460 of JimmieL. Huitt and Bruce B. McGlothlin, entitled Method and Product, led Nov.2.4, 1964, now abandoned.

This invention relates to a method for controlling oil producing sandformations penetrated by a well bore and it particularly relates to anovel gravel pack of high permeability and high integrity and to amethod of forming this new gravel pack.

Oil bearing sand formations frequently disintegrate and produce sand inthe well bore as a result of insuthcient cohesion between adjacent sandparticles under the conditions prevailing in the well. These formationsmay be so weakly consolidate that disintegration begins during thedrilling or well completion operations as the internal stresses createdby the weight of overburden are relieved. On the other hand a formationmay be suciently consolidated to withstand drilling and well completionoperations but will begin to produce sand after the well is brought intoproduction as the stress of the flowing reservoir fluid adds to thenatural reservoir stresses to overcome the weak binding forces betweenthe sand grains. This disintegration of weak formations is a seriousproblem. The production of sand with reservoir oil may severely erodethe production equipment including tubing, valves and pumps and mayeventually plug up the well. The oil and sand mixture produced fromthese wells requires separation above ground which represents aneconomic burden. On occasion massive cave-ins of subsurface formationsoccur; thereby damaging the well and equipment and necessitating theabandonment of the well.

Gravel packing of loosely consolidated sand formations is one approachwhich has been used with varying success for their support to preventthe sanding-up of wells and the possible destruction of the well. Inthis general method a cavity in the formation adjacent the productioncasing is formed. Gravel is then packed into this cavity. The gravelparticles are usually selected to have a relatively uniform diameterseveral times the diameter of the sand particles at a specific point,usually the 10 percentile point (the size of Athe smallest particles inthe coarsest 10 percent by weight of formation sand), in the sieveanalysis. Since the openings in the gravel pack are larger than most ofthe sand particles, reliance is placed upon the tendency of the sandgrains to bridge across the openings t-o stabilize the formation.Heretofore, it has not been possible to place a gravel pack ofpredictable permeability and flow characteristics. Newly installed packsordinarily are interlaced with voids which collapse during oilproduction. In addition, failure in bridging occurs with substantialmigration of particles into the pack accompanied by plugging of the packand a substantial reduction in its permeability. It is believed thatthese pack defects in part result from the initial inclusion in the packof irregular sized and shaped particles.

Gravel entrained in a carrier liquid is substantially fractured andbroken up in the process of pumping it under pressure into a gravelpack. Thus, an ordinary gravel of uniform size above ground will enterthe pack intermixed with irregular fragments or lines from broken gravelto form a mixed pack of low permeability and interlaced with voidsresulting from bridging of the jagged fragments. The void zones presentno support to the formation and permit the production of formation sandin the well while the remainder of the pack of initial low permeabilityis easily plugged with formation lines for a further reduction ofpermeability.

We have now discovered, contrary to prior experiences, that gravel packscan be produced having a good and predictable permeability andcharacterized by a uniformity without voids or plugs. In addition, wehave discovered that these packs will maintain their integrity while inproduction over long periods of time. Furthermore, these gravel packswill maintain their integrity when subjected to pressures approximatingthe overburden pressure and support the formation against movement intothe cavity under this large hydrostatic pressure. As used herein highintegrity of the gravel pack refers to its substantially completefulfillment of the characteristics and functions projected for it. It ischaracterized by a controlled uniformity of gravel size withoutfragments, voids, plugs or other nonuniformities which characterizeordinary gravel packs. It is further characterized by a predetermined,good permeability which remains substantially constant over long usewithout plugging or physical breakdown regardless of the pressure towhich it is subjected.

Our gravel pack is formed of graded special hardened glass pelletsdeposited in layers concentric with the well axis. The initial layeradjacent the cavity wall is formed with the smallest of the uniformlysized pellets. Each successive layer in a direction towards the wellbore axis is formed from glass pellets of increasing size. By thisarrangement we are able to combine the high filtering advantages of thesmall pellets contacting the formation with the high permeability of therelatively large pellets adjacent the production tubing with its higherflow rates to form a body of relatively high overallpermeability. Whenour hardened glass spheres are pumped with a carrier liquid into thewell in accordance with our invention they enter the pack in theiroriginal form without any significant breakage. Thus, the optimum, apack of spherically shaped, uniformly sized pellets is now possible. Inaddition by forming the pack in layers of increasing particle size fromthe cavity wall to the well bore, a pack is produced having anincreasing permeability in the zones of increasing ow rate of reservoiroil. An increase in size of the pore spaces of our pack from layer tolayer towards the well bore is a characteristic or our pack. As aresult, any formation fines which by chance migrate into the pack willbe carried through to the well without being trapped within the pack toform plugs. A further advantage of using these hardened glass pellets isthat a pressure approximately the overburden pressure may be appliedradially t0 the pack for complete support of the formation againstinward movement and cave-in without fracturing the individual glasspellets thereby avoiding a mixed pack with the ultimate destruction ofthe packs effectiveness. In order to produce the superior pack describedherein, the pellets should have a sphericity and roundness of at least0.8 and preferably as close to 1.0 as is possible, in accordance withthe definitions of Krumbein and Sloss at pages 78 through 83 inStratigraphy and Sedimentation, published by W. H. Freeman Company, 1951edition. Glass pellets having a roundness and sphericity exceeding 0.9can readily be manufactured.

A further aspect of our invention is the method of forming this novelpack. In order to form the pack in layers in a direction from theformation to the production tubing we utilize a carrier liquid whichwill leak off into the formation and deposit the glass pellets in suc`cessive layers on the wall of the cavity. Suitable leak-off liquids areoil, water or weighted water. In order to insure that the glassparticles deposit onto the wall of the cavity without settling out weprefer that their density approximates or equals the density of theleak-off liquid. This is accomplished by utilizing low density glassformulations, by incorporating minute gas bubbles in the glass duringproduction, by using high density liquids, or by a combination of thesefactors. This equalization of density is particularly desirable whenonly a relatively low flow of leak-off liquid into the formation ispossible due to the occurrence of a low permeability formation.

A more detailed explanation of our invention disclosing furtherfeatures, objects and advantages follows. Only so much of the well andapparatus necessary to illustrate our invention to those skilled in theart is shown in the accompanying drawings.

FIGURE 1 is a vertical sectional view of an open hole completed oil wellpenetrating an oil bearing formation showing a cavity in the oil bearingformation in readiness for gravel packing;

FIGURE 2 is a vertical sectional view showing the first layer of glasspellets on the cavity wall and the second layer being deposited on thefirst layer;

FIGURE 3 is a diagrammatic view partially in vertical section showingthe gravel pack after all of the special glass pellets have beenintroduced; and

FIGURE 4 is a vertical sectional view of the completed gravel pack readyfor oil production with a production liner inserted through the pack.

In FIGURE l, l0 represents the well bore penetrating overburden 11 andextending down through the oil sands 12 into the underlying formation 13a short distance. `Casing 14 extends down to the pay zone I2 and iscemented in place within the well bore using standard cementingpractices. Cavity 15 for insertion of the gravel pack may have resultedfrom caving in of the formation or preferably it is intentionally formedby a suitable technique once the need for the gravel pack has beenestablished. For example, the well bore may be enlarged into a cavity byjetting an abrasive-laden fluid against the formation. A jetting toolwith horizontally directed jet nozzles is reciprocated through theheight of the formation to form a good cavity. An advantage of producingthe cavity with a jetting tool is that the weaker portions of theformation are selectively removed. The jetting tool is removed and thecavity is ready for packing.

Prior to initiation of the gravel packing operation formationcharacteristics have been ascertained, preferably from a core sample,and the gravel pack designed, that is the quantity and sizes of glasspellets has been estimated, `and the pellets are on hand for the packingoperation. As indicated it is essential that the leak-off liquid andpellets be so adjusted in density that the rate of ow of leak-off liquidinto the formation, as determined by formation characteristics, will besufficient to hold the pellets in suspension and deposit them `againstthe cavity wall. The pellets should be within about 50 percent of thedensity of the leak-off liquid and it is preferred to insure reliabilityof the pack forming process that the density difference be between about0 to 20 percent of the liquid density.

This packing is accomplished by entraining the pellets in the liquid asit is pumped to the well head and pumping the mixture down the well. Asthe mixture reaches the cavity, the liquid will flow through the cavitywall 16 as indicated by the arrows in FIGURE 2 and disappear into theformation depositing and retaining the pellets firmly against the cavitywall. The smallest pellets are entrained in the liquid first and aredeposited against the cavity wall to form the layer 17 in FIGURE 2. Thisis followed as shown in FIGURE 3 by the deposition of a larger sizeglass pellet 18 which is packed against the first batch 17. The finaland largest size pellets are entrained in the leak-off fluid and arepacked against the previous layer 1S to fill up the entire cavity asindicated by 19.

In FIGURE 4 slotted liner 20 is washed through the pack by conventionalmeans until it contacts the bottom of the borehole. This washingoperation is conducted under overall uid pressure to hold the pack inposition. The well is now put into production. The pack as producedpossesses good permeability and support for the cavity wall. A cavityand pack may also be formed in a cased-hole completed well behind thecasing by washing out the cavity in a conventional manner and injectingthe liquid-pellet mixture through the perforations.

As illustrated the gravel pack is formed in three layers of differentlysized pellets with the smallest adjacent the cavity wall and the largestadjacent the production liner. It is possible to utilize two or morelayers of differently sized pellets, the number of layers depending inpart upon the size of the formation particles. The smaller the formationparticles, the smaller will be the glass pellets adjacent the formationfor effective control. We believe that one of the reasons that priorpacking efforts have so fre- -quently proven to be less effective thandesired is that the stable lbridging of small particles across largeopenings is not possible in an environment of shifting pressures andvarying flow rates. We do not rely on the bridging phenomenon butinstead size the pellets of our first layer so that the openings will besmaller than the formation particles. Thus, it is preferred that thepellet size of the first layer be from about 2 to 4 times the sand sizeat the percentile point on the formation sieve analysis (US. StandardSieve Series referred to throughout). At this size it is not possiblefor any significant quantity of formation sand to enter the pack and theonly particles that can be carried through the pack are some of theminute fines and particles of clay and silt. The pellets in the first orouter layer maybe as small as about mesh. Ordinarily pellets having asize in the range of 20-40 mesh or 40-6() mesh are used. For productionof an effective pack overall, it is preferred that the pellet size ineach succeeding layer be from about 2 to 6 times larger than thepreceding layer with the lower ratios producing the more effective pack.A final layer of 4 to 6 mesh size pellets will be found to be verysatisfactory for many packs, with the final size depending in part onthe particular production equipment utilized. The most effective packsresult if the pellets in each layer have a high degree of sphericity androundness and are substantially equal in size. For example, if analysissuggests a 60 mesh gravel for one of the layers, it is preferred to usea gravel that passes 50 mesh but is retained on 60 mesh 4rather than agravel that is less rigidly graded.

The leak-off liquid is preferably a hydrocarbon or water solution. Ifwater is used it is possible to control its density by dissolving asuitable weighting agent in the water. It is important to avoid fineparticles in the leakoff liquid that would deposit onto the cavity walland render it impermeable. In order to prevent the glass pellets fromsettling out to the bottom of the cavity rather than depositing onto thecavity wall, it is desirable to adjust the pellets and liquid densitiesto be similar or equal. If

of glass on an inclined the permeability of the formation to theleak-olf liquid is low, the ow of leak-off liquid into the formationwill be low. In this -case it is essential that the relative densitiesbe closely adjusted to prevent the segregation of the glass pellets inthe well. The density of the glass can be adjusted down to about l.5 byformation control. The density of the glass can also be adjusted byentraining minute gas bubbles in the glass in its manufacture. It isimportant in this instance that the pellets are formed without surfacebubbles since these introduce a structural weakness into the pellets.The more useful hydrocarbons are lighter than water. If a heavierleak-off liquid is desired, water itself or preferably a weighted wateris used. Sodium chloride, calcium chloride, zinc chloride and otherwellknown additives are useful for increasing the density of water. Inthis manner densities up to about twice the density of water itself areattainable. In addition, treating acids or other chemicals may beincorporated into the liquid for concurrent acidiz'ing or otherformation treatment. The selection of the density-modifying additive isdependent both on cost, on well characteristics and on the density ofthe solution desired. Thus, by adjusting the densities it is readilypossible to produce a nonsegregating mixture.

In order to make an effective pack of high integrity according to ourinvention it is necessary to utilize uniformly sized pellets and getthem into the pack witho-ut significant breakage. The glass pellets arewithdrawn from a storage unit at the well head 'by a screw conveyer andinjected into the desired liquid in `a mixing tank. The liquid with thepellets entrained therein then passes through one or more centrifugalpumps to a final high pressure pump all with associated valves to beforced through the well bore under pressure. We have discovered thatthis multiple pumping operation involves significant breakage ofconventional gravel packing materials. A special glass pellet whoseproduction will now be described is used to prevent significant breakagein this pumping operation and permit the placement of our multilayerpack of high integrity.

The special gravel packing agents required in this invention can beprepared from any of the usual types of glass such as soda-lime, lead,boro-silicate, and high silica glasses or they can be prepared fromslags and other low density formulations. Particles of soda-lime glassare heated near its softening temperature in excess of 1800o F. suchthat any bubbling of gas in the glass either has not begun or hasceased, and then the particles are rapidly quench in a tluid at atemperature below about 900 F.

T he quenching fluid may be a gas or a liquid having a viscosity greaterthan water; however, water alone is not suitable. The temperature fromwhich the glass is quenched will have a strong influence on the abilityof the glass particles to resist crushing and deformation undercompressive loads. For example, ordinary bottle glass spheres quenchedin accordance with this invention from a temperature of 2200 F. in a uidto a temperature below 900 F. have a higher strength than similarparticles quenched from 1800 F., and it is preferred that the packingagent be prepared by quenching glass particles from a temperature above2000 F. One of the advantages of the rapid quenching process is that itallows the production of glass particles -capable of withstanding highercompressive loads than ordinary soda-linie bottle glass.

The particles are formed by dropping molten globules heated surface of amaterial, such as graphite, which is not wetted by the glass. Theglobules of the glass roll along the surface of the graphite to formspheres and then roll from the surface into the quenching medium.Suitable quenching media are hydrocarbon oils such as SAE l0, SAE 20,and SA-E 30 motor oils, viscous aqueous solutions such as aqueous waterglass solutions, starch solutions, soap solutions or aqueous solutionsof ethylene glycol, and mixtures of oils and melted greases and fats.The quenching medium is at a temperature below about 400 F., and ispreferably at room temperature. After the temperature of the glassparticles has reached a temperature below 900 F., further cooling may beaccomplished in any convenient manner. It is not necessary that thecooling from 900 F. to lower temperatures be rapid. The particles arethen Sorted and graded as to size, density, sphericity and roundness.The particles are highly rounded, ordinarily having a roundness andsphericity well about 0.9.

The novel packing agents are strong and hard, but brittle in that theyfail in tension when subjected to a compressive load. A convenientmeasure of the properties of the particles of the packing agent is givenby the ratio of L/D2, where L is the maximum compressive load in poundsa single particle can carry and D is the diameter of the particle ininches. The ratio of L/D2 is determined by placing a measured singleparticle of packing agent between hard steel plates and pressing theplates together with a known force that is increased until the particleruptures. These packing agents are in general characterized Iby an L/D2ratio exceeding 50,000 when the particles are tested between steelplates having a hardness of 35 Rockwell C. The lower density packingagents prepared from slags and other low density formulations have anL/D2 ratio of 30,000 p.s.i. or more. An excellent grade of packing sandin 10-20 mesh size has an L/D2 ratio of approximately 10,000. Ordinarysand has an L/D2 ratio of approximately 4,000. Ordinary glass sphereshave an L/D2 ratio approximately the same as the excellent grade ofpacking sand. These special glass packing agents are furthercharacterized by being unyielding or substantially non-deformable whensubjected to compressive loads; hence, they retain substantially theirinitial dimensions until they fail. Other suitable processes includingchemical tempering of glass pellets may be used for producing specialgravel packing agents, useful hereunder, having a suitably high L/D2ratio and the other desirable attributes.

As previously indicated it is not only necessary to get uniformly sizedpellets into the pack but in addition it is necessary that they do notbreak up in the pack over a long period of time. The formation may besubjected to a large overburden pressure which is translated into ahorizontal component when the borehole and cavity have been produced.This horizontal component of the overburden pressure together with thepressure of the oil being produced will cause a substantial, continuouspressure inwardly towards the well bore. In some instances it isdesirable to counteract this pressure by exerting an opposing positivepressure against the pack. An expandable liner such as described byHildebrandt in U.S. Patent No. 2,998,065 can be used to exert a pressureon the pack and support the cavity wall. The pressure placed on the packto support the formation will be from about 0.5 to 1.1 times theoverburden pressure. In order to withstand the very substantialunderground pressures that may exist and still retain pack integrity andpermeability, it is desirable to use these special glass pellets.Otherwise the pellets may gradually fracture to form a dense, pluggedup,substantially non-permeable pack. When the gravel pack is located inshallow formations not subject to higr overburden pressures, glasspellets of less strength, such as the low density pellets having an L/D2of 30,000 p.s.i. or more can be used.

A further advantage of using these hardened glass pellets is theirability to withstand elevated temperatures and their maintenance ofstrength and corrosion resistance in the presence of acids, brine,caustic and other chemicals. Ordinary glass is generally considered tobe about half as strong in the presence Iof water as it is in the drystate. Our special glass pellets are not affected by the presence ofwater. Since water in the well is almost a certainty, this advantage isof substantial signicance in maintaining pack integrity. Furthermore,chemical or heat treatment of the formation is possible withoutaffecting the pack. For example, corrosive acids and brine have 7ngliglixble effect on the pellets at temperatures as high as 3 0 In aspecific example of my invention a borehole is drilled and an oilbearing zone discovered between 3476 feet and 3501 feet. A core samplereveals that the formation is a weakly consolidated sandstone having apermeability of about 200 millidarcies. Ninety percent of its particlesare retained on a 270 mesh sieve, U.S. Standard Sieve Series. It isdecided to use our gravel pack as a protective measure against formationdisintegration and production of sand with the reservoir oil.

A casing is lowered into the well to a depth of 3470 feet and iscemented in place. The cement is then drilled out through a depth of3510 feet, the bottom nine feet being in consolidated rock. A cavityapproximately three feet in diameter is formed in the pay zone byjetting a sand-laden water mixture from a jetting tool reciprocatedthrough the productive interval. An aqueous leak-off solution having aspecific gravity of 1.9 is prepared from Water, calcium chloride andzinc chloride. Seventy to eighty mesh hardened slag-type glass pelletshaving a specific gravity of approximately 2.3 are entrained in theleak-olf liquid in an amount of five pounds per gallon and the mixtureis pumped down the tubing at a pressure not exceeding 2500 p.s.i. After7000l pounds of these 70 to 80 mesh pellets have been deposited againstthe cavity wall, 5000 pounds of to 25 mesh pellets are entrained in theleak-off liquid in like manner and pumped into the cavity. This isfollowed up by a final 6000 pound batch of 6 to 7 mesh pellets to tillthe cavity. A fluid pressure of about 1600 p.s.i. is maintained in thewell at the formation to hold the pack in place as a six-inch diameterslotted liner is washed into position through the 6 to 7 mesh pelletportion of the pack. Inspection of the pellets that are washed to thewell head during the placement of the liner reveals that there is nosignificant quantity of crushed pellets in the pack. The pack consistsof an outer four inch thick layer of 70 to 80 mesh pellets, a middlelayer four inches thick of the 20 to 25 mesh pellets and an interiorlayer seven inches thick of the 6 to 7 mesh pellets. The well is thenput into production in the conventional manner. It is ascertained frompressure build-up data that pack permeability exceeds 100 darcies whichindicates that the pack possesses good integrity with no crushedpellets, voids or plugs.

By this invention a gravel pack of high permeability and integrity maybe produced. The pack will support poorly consolidated formationsagainst pressures as great as the overburden pressure or greater andmaintain its high permeability for an extended period of time. lt may beused in a great variety of situations to protect weakly consolidatedformations against disintegration, provided that the formation possessessufficient permeability to place the pack in the manner required by thisprocess.

We claim:

1. In the production of oil from an oil bearing formation containing apacked cavity between the production tubing and formation to prevent themigration of formation particles into the well, the method of forming apack in the cavity which comprises entraining in a leak-off liquid a rstbatch of discrete, rigid, substantially spherical glass pelletsessentially uniform in size and specially produced to have an L/D2 ratioof at least about 50,000 Where L is the maximum compressive load inpounds that the pellet can carry and D is the diameter of the particlein inches, pumping the leak-off fluid and glass pellet mixture into thewell under pressure to cause the leak-off uid to enter the formationthrough the cavity wall and deposit the glass pellets as a layer on thewall of the cavity, entraining in a further portion of the leak-offliquid at least one more batch of glass pellets identical to thepreceding batch -but of a uniform larger size than the preceding batch,and depositing them in the cavity against the preceding batch of glasspellets to form a pack of high integrity of graded spherical glasspellets of increasing f8 size in -a direction from the cavity wall tothe borehole.

2. A method in accordance with claim 1 in which a pressure of betweenabout 0.5 and about 1.1 times the overburden pressure is appliedradially against the pack without breakage of the pellets to force thepack against the cavity wall and support the formation against inwardmovement.

3. A method in accordance with claim 1 in which the pellets in the firstlayer adjacent the cavity Wall are from 2 to 4 times the 90 percentilepoint on the formation sieve analysis and the pellets in each succeedinglayer are from 2 to 6 times larger than the pellets in the immediatelypreceding layer.

4. A method in accordance with claim 1 in which the density differencebetween the glass pellets and the leakoff liquid is adjusted to be nogreater than about 50 percent and the leak-off liquid is a weightedwater solution.

5. A method in accordance with claim 4 in which the glass pellets areformed to contain a multiple of minute gas bubbles.

6. A method in accordance with claim 4 in which the glass pellets are aspecially hardened, low density, slagtype glass.

7. A method in accordance with claim 4 in which the density of the glasspellets and the leak-oft liquid is substantially equal.

8. A gravel pack of high integrity in a cavity formed in the oilproducing formation surrounding the production tubing in a boreholecomprising a series of at least two layers of discrete, rigid,substantially spherical glass pellets essentially uniform in size ineach layer and of increasing size from layer to layer in a directionfrom the cavity wall to the production tubing, and specially produced tohave an L/.D2 ratio of at least about 50,000 where L is the maximumcompressive load in pounds that the pellet can carry and D is thediameter of the particle in inches, said pack characterized by a highpermeability and by the absence of voids, plugs and fragmented pellets.

9. A gravel pack in accordance with claim 8 in which said productiontubing comprises an expandable liner which applies pressure laterallyagainst the pack to force the pack against the cavity wall at a pressurewhich is from about 0.5 to 1.1 times the overburden pressure.

10. A gravel pack in accordance with claim 8 in which the pellets in thefirst layer adjacent the cavity wall are from 2 to 4 times the 90percentile point on the formation sieve analysis and the pellets in eachsucceeding layer are from 2 to 6 times larger than the pellets in theimmediately preceding layer.

11. In the production of oil from an oil bearing formation containing apacked cavity between the production tubing and formation to prevent themigration of formation particles into the well, the method of forming apack in the cavity which comprises entraining in a leak-off liquid aiirst batch of discrete, rigid, substantially spherical glass pelletsessentially uniform in size, pumping the leak-off fluid and glass pelletmixture into the well under pressure to cause the leak-off fluid toenter the formation through the cavity wall and deposit the glasspellets as a layer on the wall of the cavity, entraining in a furtherportion of the leak-ofi liquid at least one more batch of glass pelletsof a uniform larger size than the preceding batch, and depositing themin the cavity against the preceding batch of glass pellets to form apack of high integrity of graded spherical glass pellets of increasingsize in a direction from the cavity wall to the borehole.

12. A method in accordance with claim 11 in which the density differencebetween the glass pellets and the leak-off liquid is adjusted to be nogreater than about 50 percent and the leak-olf liquid is a weightedwater solution.

13. A method in accordance with claim 11 in which the glass pellets arehardened, low-density, slag-type glass.

14. A method in accordance with claim 11 in which the pellets in thefirst layer adjacent the cavity wall are from 2 to 4 times the 90percentile point on the formation sieve analysis and the pellets in eachsucceeding layer a-re from 2 to 6 times larger than the pellets in theirnmediately preceding layer.

15. A gravel pack of high integrity in a cavity formed in anoil-producing formation surrounding production tubing in a boreholelcomprising a series of at least two layers of discrete, rigid,substantially spherical glass pellets substantially uniform in size ineach layer and of increasing size from layer to layer in a directionfrom the cavity Wall to the production tubing, said pack characterizedby a high permeability and by the absence of voids, plugs and fragmentedpellets, and said production tubing comprising an expandable liner whichapplies pressure laterally against the pack to force the pack againstthe cavity wall at a pressure from about 0.5 to 1.1 times the overburdenpressure.

References Cited CHARLES E. OCONNELL, Primary Examiner.

DAVID H. BROWN, Examiner.

